Zirconium alloys are used as a cladding material in most nuclear reactors worldwide due to properties uniquely suited to the operating environment of a reactor. In this thesis, density functional theory (DFT) simulations were conducted to investigate the behaviour of fission product dopants in the inner cladding oxide, and to examine the role this layer plays in limiting corrosion in the context of pellet-cladding interaction (PCI).

Simulations in undoped monoclinic, tetragonal and cubic ZrO$_{2}$ yielded structure properties in addition to intrinsic defect energies, volumes and defect equilibria. Fully-charged Schottky defects \{2\ch{V_{O}^{**}}:\ch{V_{Zr}^{''''}}\}$^{\times}$ had the smallest formation energies in each phase, followed by O Frenkels and then Zr Frenkels. Defective cubic ZrO$_{2}$ simulations are sensitive to finite-size effects, and would often break symmetry or collapse into the tetragonal phase when defect clusters were introduced. Free energy calculations predicted a transition from monoclinic to tetragonal as temperature was increased, but not from tetragonal to cubic.

Iodine defects adopt oxidation states of +1 (\ch{I_{O}^{***}}, \ch{I_{i}^{*}} and \ch{I_{Zr}^{'''}}) or -1 (\ch{I_{O}^{*}}) in ZrO$_{2}$ , with fewer defects in the 0 oxidation state (\ch{I_{O}^{**}}). At high oxygen partial pressures ($p_{O_{2}}$), iodine defects in tetragonal ZrO$_{2}$ fall significantly. Iodine defects in monoclinic ZrO$_{2}$ changed by small amounts as $p_{O_{2}}$ was increased. This demonstrated competition between iodine and oxygen in ZrO$_{2}$, and that it is dependent on both $p_{O_{2}}$ and phase. High $p_{O_{2}}$ in the tetragonal phase provides the greatest barrier to iodine ingress.

During reactor power ramps, the quantity of fission products implanted in the oxide layer will increase. Decay rates of Te and I isotopes were found to be commensurate with time to failure in irradiation tests. Defect equilibria and volumes of Te, I, Xe and Cs were obtained in tetragonal ZrO$_{2}$ to investigate the effect of nuclear transmutation while dopant atoms are present. Defect evolution on the O site is predicted to be \ch{Te_{O}^{**}} -> \ch{I_{O}^{*}} -> \ch{Xe_{O}^{**}} -> \ch{Cs_{O}^{**}}. On the Zr site, Brouwer diagrams predict \ch{Te_{Zr}^{'''}} -> \ch{I_{Zr}^{'''}} -> \ch{Xe_{Zr}^{''''}} ->\ch{Cs_{Zr}^{'''}}. These defects have large defect volumes and will generate stresses which may promote crack formation.